Azimuth meter

ABSTRACT

To provide a thin and small area azimuth meter. A plane coil and at least two groups of thin film magneto resistive elements are arranged. Each of the groups of thin film magneto resistive elements constitutes an MR bridge and detects and outputs two perpendicular components of the earth magnetism, and bearing information is obtained based on the output values.

TECHNICAL FIELD

The present invention relates to a flat azimuth meter or bearing sensorhaving a plane coil laminated with thin film magneto resistive elements(hereinafter referred to as “magneto resistive elements”) and to a smalland light azimuth meter suitable for mobile devices.

BACKGROUND ART

When a current is applied to a magneto resistive element in a directionof an easy axis of magnetization, and at the same time, a magnetic fieldis applied in a direction perpendicular thereto, an electric resistancein the current direction has a magneto-resistance effect, that is, it isreduced depending on a magnetic field strength. A relationship betweenthe electric resistance (hereinafter referred to as “resistance”) andthe applied magnetic field strength can substantially be shown as inFIG. 20.

Assuming Hk denotes a saturation magnetic field, when a biasing magneticfield on the order of ½·Hk is applied to a magneto resistive element,there is a substantially linear relationship between an externalmagnetic field H and the resistance R. An external magnetic field can bemeasured by using the linear relationship between the external magneticfield H and the resistance R when a certain biasing magnetic field isapplied. Then, when each of two components orthogonal to each other ofthe earth magnetism is detected by two groups of magneto resistiveelements that an appropriate bias is applied to, bearings can bemeasured at a measuring point.

There is used an azimuth meter or a bearing sensor comprising an MRbridge constituted by four magneto resistive elements 91, 92, 93, and 94that are orthogonal to each other as shown in FIG. 21, and two biascoils 101 and 102 that are wound around a holder mounted outside of themagneto resistive elements so that two orthogonal biasing magneticfields can be applied both at an angle of 45 degrees with respect to thecurrent directions of the magneto resistive elements. FIG. 22 is aschematic cross-sectional view thereof, and FIG. 23 is a perspectiveview thereof.

In measurement of bearings, a +x-direction bias is applied by one biascoil 101 (referred to as an x-direction coil) to the four magnetoresistive elements 91, 92, 93, and 94 constituting the MR bridge tomeasure an intermediate potential difference among the magneto resistiveelements, and then, a −x-direction bias is applied by the same bias coil101 to the magneto resistive elements to measure the intermediatepotential difference among the magneto resistive elements. A differencebetween the intermediate potential differences measured when the+x-direction bias is applied and when the −x-direction bias is appliedis proportional to sin θ, the angle θ being an angle between thehorizontal component of the earth magnetism and the x-axis.

Next, a +y-direction bias is applied by the other bias coil 102(referred to as a y-direction coil) to the four magneto resistiveelements 91, 92, 93, and 94 constituting the MR bridge to measure anintermediate potential difference among the magneto resistive elements,and then, a −y-direction bias is applied by the same bias coil 102 tothe magneto resistive elements to measure the intermediate potentialdifference among the magneto resistive elements. A difference betweenthe intermediate potential differences measured when the +y-directionbias is applied and when the −y-direction bias is applied isproportional to sin(π/2−θ), that is, cos θ.

From the y-directional output Vy and the x-directional output Vx, thebearings can be measured as the direction θ of the horizontal componentof the earth magnetism as follows:

θ=tan⁻¹(Vx/Vy).

However, the relationship between the magnetic field applied to themagneto resistive element and the resistance practically involves ahysteresis as shown in FIG. 24, rather than FIG. 20. When the appliedmagnetic field strength H is increased, it reaches a level of saturationvia the upper curve in FIG. 24, and when it is decreased from the level,it traces the lower curve.

Therefore, when measuring bearings, the saturation magnetic field isapplied before the application of the biasing magnetic field inconsideration of the hysteresis.

For example, as disclosed in Japanese Patent Laid-Open No. 5-157565,when measuring bearings using the azimuth meter composed of the magnetoresistive elements and two orthogonal bias coils as described above, thesaturation magnetic field Hk is applied in +x direction, and then theintermediate potential difference between the magneto resistive elementsis measured while applying the +x-direction biasing magnetic field Hb.Then, the saturation magnetic field −Hk is applied in −x direction bythe same bias coil, and then the intermediate potential differencebetween the magneto resistive elements is measured while applying the−x-direction biasing magnetic field −Hb. The difference between theintermediate potential differences at the time of applications of the+x-direction bias and the −x-direction bias thus obtained is defined asan x-direction output Vx.

Then, the saturation magnetic field is applied in the +y direction bythe other bias coil, and then the intermediate potential differencebetween the magneto resistive elements is measured while applying the+y-direction biasing magnetic field. Then, the saturation magnetic fieldis applied in the −y direction by the same bias coil, and then theintermediate potential difference between the magneto resistive elementsis measured while applying the −y-direction biasing magnetic field. Thedifference between the intermediate potential differences at the time ofapplications of the +y-direction bias and the −y-direction bias thusobtained is defined as an y-direction output Vy. Based on the Vx and Vy,bearings are measured in the manner as described above.

The orthogonal four magneto resistive elements assembled into the MRbridge described above may be formed as zigzag magneto resistiveelements formed by etching a Ni-based alloy film deposited on a ceramicsubstrate. Thus, the magneto resistive elements can be quite small andthin. However, since the two bias coil wound around them in x directionand y direction are provided outside the magneto resistive elementbridge, the azimuth meter has, at the smallest, a thickness of the orderof 3 mm and an area of the order of 10 mm×10 mm.

In the procedure of measuring bearings explained in the abovedescription, it is required to carry out measuring four times becausethe bias is applied in +x direction and −x direction by the x-directioncoil, the bias is applied in +y direction and −y direction by they-direction coil, and then calculation is carried out.

Furthermore, in order to eliminate the effect of the hysteresis, beforethe biasing magnetic field is applied, the saturation magnetic field ofthe same direction as that of the biasing magnetic field is applied.After the application of the saturation magnetic field, application ofthe biasing magnetic field of the same direction tends to make thegradient of the curve for the resistance of the magneto resistiveelement, and the magnetic field be decreased, so that the output to bemeasured becomes low.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an azimuth meter or abearing sensor of a significantly reduced thickness and area.

Furthermore, another object of the present invention is to provide anazimuth meter or a bearing sensor in which the number of applications ofa current to a coil and the number of measurements are less than before.

For example, an azimuth meter according to the invention comprises: aplane coil wound into a rectangular shape; and at least two groups ofthin film magneto resistive elements disposed substantially parallel tothe plane of the plane coil, in which each of said groups of magnetoresistive elements constitutes an MR bridge of an even number of magnetoresistive elements electrically connected to each other and detects andoutputs two perpendicular components of the earth magnetism, and bearinginformation is obtained based on the output values, wherein the azimuthmeter further comprises: means of passing a current in a predetermineddirection through the plane coil to apply thereto a magnetic field thatis equal to or higher than a saturation magnetization of said magnetoresistive elements, applying a constant biasing magnetic field in thedirection opposite to the direction, applying a magnetic field equal toor higher than the saturation magnetization of the magneto resistiveelements in the direction opposite to said predetermined direction, andthen applying a biasing magnetic field in the direction opposite to thelatter direction; and means of passing a magnetic field measuringcurrent through the groups of the thin film magneto resistive elementsconcurrently with the applications of said biasing magnetic fields.

Preferably, a circuit is arranged so that, when one power supply is usedto apply the magnetic field equal to or higher than the saturationmagnetization of the magneto resistive elements, a discharge voltage ofa capacitor having been previously charged by a shunt current from thepower supply is superimposed to the voltage applied to the plane coil.

Preferably, an angle β formed between a longitudinal direction of eachmagneto resistive element and a side of the plane coil in the vicinityof the magneto resistive element satisfies a relation of sin β×cos β≠0,and an applied magnetic field characteristic in the vicinity of a regionwhere the electrical resistance variation of the magneto resistiveelement in response to the applied magnetic field is the minimum isused.

Preferably, one of the groups of thin film magneto resistive elements isconstituted by two pairs of magneto resistive elements, the magnetoresistive elements in each pair being disposed intersecting oppositesides of said rectangular coil and being electrically connected to eachother, the other of the groups of thin film magneto resistive elementsis constituted by two pairs of magneto resistive elements, the magnetoresistive elements in each pair being disposed intersecting oppositesides, different from said sides, of said rectangular coil and beingelectrically connected to each other, and the longitudinal directions oftwo magneto resistive elements disposed on a same side are substantiallyperpendicular to each other.

Preferably, the angle β is any of about 45 degrees, about 135 degrees,about 225 degrees and about 315 degrees. Preferably, the variation ofthe angle at which the longitudinal direction of each magneto resistiveelement intersects the side of the rectangular coil falls within a rangeof ±5 degrees. If necessary, in the case where the two perpendicularcomponents of the earth magnetism are detected by each of the groups ofmagneto resistive elements and output therefrom, and the bearinginformation is obtained based on the output values, a circuit may beadditionally provided which outputs a difference between an outputobtained when a bias is applied in a positive direction and an outputobtained when a bias is applied in a negative direction.

A procedure of passing a current through the plane coil in apredetermined direction to apply thereto a magnetic field that is equalto or higher than a saturation magnetization of said magneto resistiveelements, applying a constant biasing magnetic field in the oppositedirection, and then measuring the resulting magnetic field to obtain anoutput value and a procedure of applying a magnetic field equal to orhigher than the saturation magnetization of the magneto resistiveelements in the direction opposite to said predetermined direction,applying a biasing magnetic field in the opposite direction, and thenmeasuring the resulting magnetic field to obtain an output value may beperformed two or more times, and the bearing information may be obtainedbased on the output values.

In the azimuth meter according to the invention, an even number ofmagneto resistive elements are electrically connected to each other toconstitute an MR bridge. For example, in the “MR bridge” in theinvention, magneto resistive elements A and B are disposed on oppositesides of a plane coil wound in a rectangular shape and connected inseries, a magneto resistive element C perpendicular to the magnetoresistive element A is disposed on the same side as the magnetoresistive element A, a magneto resistive element D perpendicular to themagneto resistive element B is disposed on the same side as the magnetoresistive element B, and the magneto resistive elements C and D areconnected in series. And, the bridge is arranged to output a potentialdifference between an output V1 at the midpoint between the magnetoresistive elements A and B and an output V2 at the midpoint between themagneto resistive elements C and D. According to the invention, twogenerally perpendicular components of the earth magnetism are detectedby each of the groups of magneto resistive elements, and the bearinginformation is obtained based on the outputs thereof. This can reducethe effect of a hysteresis of an applied magnetic field on a resistance,eliminate the noise in the output and increase the absolute value of theoutput.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic plan view of EXAMPLE of the invention;

FIG. 2 is an exploded perspective view of EXAMPLE of the invention;

FIG. 3 is an example of a circuit diagram of magneto resistive elementssuitable for EXAMPLE of the invention;

FIG. 4 is a graph showing a relationship between a resistance and anexternal magnetic field strength;

FIG. 5 is an explanatory view of operation of magneto resistive elementsaccording to the invention;

FIG. 6 is an explanatory circuit diagram for driving the azimuth meteraccording to the invention;

FIG. 7 is a timing chart of driving of the azimuth meter of theinvention;

FIG. 8 is graphs showing voltage wave and coil current wave of thedriving circuit for the azimuth meter of the invention;

FIG. 9 is a timing chart of driving of the azimuth meter of theinvention;

FIG. 10 is another circuit diagram for driving the azimuth meteraccording to the invention;

FIG. 11 is a timing chart of driving of the azimuth meter of theinvention;

FIG. 12 is a graph showing measured coil current wave of the azimuthmeter of the invention;

FIG. 13 is an enlarged graph showing measured coil current wave of theazimuth meter of the invention;

FIG. 14 is another circuit diagram for driving the azimuth meter of theinvention;

FIG. 15 is a graph explaining circuit characteristics of the invention;

FIG. 16 is a circuit diagram of analog output of the invention;

FIG. 17 is another circuit diagram of analog output of the invention;

FIG. 18 is a perspective view of a mobile phone with navigationinstalled with the azimuth meter of the invention;

FIG. 19 is a perspective view of a conventional mobile phone withnavigation;

FIG. 20 is a typical graph explaining a relationship between an electricresistance and an applied magnetic field strength;

FIG. 21 is an explanatory circuit diagram of an MR bridge for a typicalazimuth meter;

FIG. 22 is a schematic cross-sectional view of the MR bridge for theconventional azimuth meter;

FIG. 23 is a perspective view of the conventional azimuth meter; and

FIG. 24 is a graph showing a hysteresis involved in the relationshipbetween the electric resistance and the applied magnetic field strength.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a plan view of an azimuth meter of EXAMPLE of theinvention. In FIG. 1, reference numeral 1 denotes a tetragonal planecoil consisting of several tens of turns. On the same side of the planecoil, under the plane coil in this drawing, four pairs of magnetoresistive element pairs 2, 3, 4, and 5 are provided in a plane parallelto the plane coil. The magneto resistive element pairs 2, 3, 4, and 5are constituted by two magneto resistive elements 21 and 22, 31 and 32,41 and 42, and 51 and 52, respectively.

The longitudinal direction of the magneto resistive element 21, which isone of the magneto resistive element pair 2, crosses only a side 11 ofthe plane coil 1 at an angle of about 45 degrees. The longitudinaldirection of the magneto resistive element 22, which is the other of themagneto resistive element pair 2, crosses only the opposed side, thatis, a side 12 of the plane coil 1 at an angle of about 45 degrees. Thelongitudinal direction of the magneto resistive element 21 issubstantially perpendicular to the longitudinal direction of the magnetoresistive element 22, and each of these magneto resistive elements 21and 22 is connected to the other at one terminal thereof (a terminal onthe inner side of the plane coil 1 in this EXAMPLE). Similarly, as forthe other magneto resistive element pairs 3, 4, and 5, the longitudinaldirections of the magneto resistive elements 31, 41, and 51 cross onlythe sides 12, 13, and 14, respectively, each of which is one side of theplane coil 1, and the longitudinal directions of the magneto resistiveelements 32, 42, and 52 cross only the opposed sides 11, 14, and 13 ofthe plane coil 1, respectively, at an angle of about 45 degrees. Inaddition, the longitudinal directions of the magneto resistive elements31, 41, and 51 are perpendicular to the longitudinal directions of theirrespective associated magneto resistive elements 32, 42, and 52. And,each of the magneto resistive elements 31 and 32 is connected to theother at one terminal thereof (a terminal on the inner side of the planecoil 1 in this EXAMPLE), each of the magneto resistive elements 41 and42 is connected to the other at one terminal thereof (a terminal on theinner side of the plane coil 1 in this EXAMPLE), and each of the magnetoresistive elements 51 and 52 is connected to the other at one terminalthereof (a terminal on the inner side of the plane coil 1 in thisEXAMPLE). Furthermore, the longitudinal directions of the two magnetoresistive elements 21 and 32 both crossing the side 11 of the plane coil1 are perpendicular to each other. Similarly, the longitudinaldirections of the two magneto resistive elements 22 and 31 both crossingthe side 12, the longitudinal directions of the two magneto resistiveelements 41 and 52 both crossing the side 13, and the longitudinaldirections of the two magneto resistive elements 42 and 51 both crossingthe side 14 are perpendicular to one another.

In this azimuth meter, the magneto resistive elements are provided on asubstrate and further the plane coil is provided. The substrate is 0.7mm thick. The thin film portion including the magneto resistive elementsand plane coil disposed on the substrate is 40 to 50 micrometers thick.The substrate has a size of 3 mm×4 mm.

For a better understanding of EXAMPLE of the azimuth meter shown in FIG.1, FIG. 2 is a schematic exploded perspective view thereof, FIG. 3 is acircuit diagram thereof, and FIG. 6 is an explanatory circuit diagramfor driving the azimuth meter according to the invention. As seen fromFIG. 2, when a direct current is passed through the plane coil 1, a DCmagnetic field is generated in a direction from the inside to theoutside of the coil or vice versa in a plane parallel to the plane coil,so that the DC magnetic field is applied to the magneto resistiveelement pairs. With reference to FIG. 3, when a current Ib passesthrough the plane coil 1 in a clockwise direction, a magnetic field in xdirection is applied to the magneto resistive elements 21 and 32, amagnetic field in the −x direction is applied to the magneto resistiveelements 22 and 31, a magnetic field in the y direction is applied tothe magneto resistive elements 41 and 52, and a magnetic field in the −ydirection is applied to the magneto resistive elements 42 and 51. If acurrent −Ib passes through the plane coil 1 in the opposite direction, amagnetic field in the opposite direction is applied to each of themagneto resistive elements. That is, the four magneto resistive elements21, 22, 31 and 32 constitute an MR bridge, while the four magnetoresistive elements 41, 42, 51 and 52 constitute another MR bridge.

If, when a current is passed through a magneto resistive element in thelongitudinal direction, an when a magnetic field is applied in adirection perpendicular to the longitudinal direction in the magnetoresistive element plane, the resistance of the magneto resistive elementis decreased depending on the magnetic field strength as shown in FIG.20 and causes a hysteresis depending on the direction of the appliedmagnetic field as shown in FIG. 24.

If the magneto resistive element crosses the side of the plane coil atan angle of 45 degrees as in the present invention, the externalmagnetic field is applied in a direction of 45 degrees with respect to adirection perpendicular to the longitudinal direction. At this time, themagneto resistive element has a shape magnetic anisotropy in thelongitudinal direction, so that this situation can be considered to beequivalent to the situation in which a composite vector of the magneticshape anisotropy magnetic field and the external magnetic field isapplied thereto. Therefore, the relationship between the externalmagnetic field and the resistance when the external magnetic field isapplied to the magneto resistive thin plate is represented by the graphshown in FIG. 4. FIG. 4 shows the variation (a) in the resistance in thecase where while a strong magnetic field is applied in a positivedirection, the strength of the applied magnetic field is graduallyreduced, and the reversal variation (b). Since the resistance has theminimum value while the applied magnetic field is negative, when anegative magnetic field of a predetermined strength is applied after apositive strong magnetic field is applied and reduced, the variationratio of the resistance with respect to the variation in the appliedmagnetic field becomes the largest. In the case where while the strongmagnetic field is applied in the negative direction and the appliedmagnetic field is gradually increased, the variation ratio of theresistance with respect to the variation in the applied magnetic fieldbecomes the largest when a positive magnetic field of a predeterminedstrength is applied.

Therefore, in the present invention, when measuring bearings by usingthe azimuth meter of EXAMPLE shown in FIGS. 1 through 3, a directcurrent is passed through the plane coil 1 in the clockwise direction inFIG. 3 to apply to the magneto resistive elements 21 to 52 a DC magneticfield enough to saturate a magnetic field of each of the magnetoresistive elements 21 to 52 in the direction perpendicular to thelongitudinal direction, and a direct current of a predetermined strengthis passed through the plane coil 1 in the opposite direction(counterclockwise direction in FIG. 3) to apply to the magneto resistiveelements a biasing DC magnetic field in a direction perpendicular to thelongitudinal direction, and during the application of the biasing DCmagnetic field, a voltage Vcc for measuring bearings is applied betweenthe other terminals of the magneto resistive elements of the magnetoresistive element pairs to take out an intermediate potential from theconnected terminals of the magneto resistive element pairs. When thedirect current is passed in the clockwise direction in FIG. 3 to applyto the magneto resistive elements 21 to 52 the DC magnetic field enoughto saturate the magnetic field of each of the magneto resistive elements21 to 52 in the direction perpendicular to the longitudinal direction,all of the magneto resistive elements enter a state shown in the rightend of FIG. 4. The direct current is reduced or turned off, and a DCmagnetic field caused by a direct current of a predetermined strength ina direction opposite to the direct current, in particular,counterclockwise direction in FIG. 3, that is, a magnetic field of amagnitude at which the variation ratio of the resistance with respect tothe applied magnetic field approaches its maximum value is applied totake out an intermediate potential from the connected terminals of themagneto resistive elements. Now, it is assumed that the magnitude of thehorizontal component of the earth magnetism is denoted by He, and theangle of the horizontal component of the earth magnetism He with respectto x axis is denoted by θ. Then, the intermediate potential output ofthe magneto resistive elements 21 and 22 is represented by:

Vcc·(½−1/(2·Rb)·βHe cos θ),

where reference symbol β denotes a variation ratio of the resistancewith respect to a magnetic field, and reference symbol Rb denotes aresistance of the magneto resistive element at the time when only thebiasing magnetic field Hb is applied.

In this EXAMPLE, since the difference between the intermediate potentialoutputs of the connected terminals of the magneto resistive elementpairs 2 and 3 is taken out as Vx in FIG. 3, the intermediate potentialoutput difference Vx is represented by:

Vx(+)=Vcc·((½−1/(2·Rb)·βHe cos θ)−(½+1/(2·Rb)·βHe cos θ))=−Vcc·1/Rb·βHecos θ.

Similarly, since the difference between the intermediate potentialoutputs of the connected terminals of the magneto resistive elementpairs 4 and 5 is taken out as Vy in FIG. 3, the intermediate potentialoutput difference Vy is represented by:

Vy(+)=Vcc·(½−1/(2·Rb)·βHe sin θ)−(½+1/(2·Rb)·βHe sin θ))=−Vcc·1/Rb·βHesin θ.

Next, a direct current is passed through the plane coil 1 in a directionopposite to that of the above description (counterclockwise direction inFIG. 3) to apply to the magneto resistive elements 21 to 52 a DCmagnetic field enough to saturate a magnetic field of each of themagneto resistive elements 21 to 52 in the direction perpendicular tothe longitudinal direction, and a direct current of a predeterminedstrength is passed through the plane coil 1 in a direction opposite tothe former direct current (clockwise direction in FIG. 3) to apply tothe magneto resistive elements a biasing DC magnetic field in thedirection perpendicular to the longitudinal direction, and during theapplication of the biasing DC magnetic field, a voltage Vcc formeasuring bearings is applied between the other terminals of the magnetoresistive elements of the magneto resistive element pairs in the samemanner as described above to take out an intermediate potential from theconnected terminals. Provided that the absolute value of the magnitudeof the applied magnetic field is substantially the same as that of theabove description, the variation ratio of the resistance with respect tothe applied magnetic field becomes maximum.

The difference between the intermediate potential outputs of theconnected portions of the magneto resistive element pairs 2 and 3 istaken out as Vx in FIG. 3, the intermediate potential output differenceVx being represented by:

Vx(−)=Vcc·1/Rb·βHe cos θ.

And, the difference between the intermediate potential outputs of theconnected terminals of the magneto resistive element pairs 4 and 5 istaken out as Vy in FIG. 3, the intermediate potential output differenceVy being represented by:

Vy(−)=Vcc·1/Rb·βHe sin θ.

The differences between the intermediate potential output differences inx direction and y direction are obtained as follows:

V in x direction =Vx(+)−Vx(−)=−2 Vcc·1/Rb·βHe cos θ;

V in y direction =Vy(+)−Vy(−)=−2 Vcc·1/Rb·βHe sin θ.

Therefore, the angle θ of the horizontal component of the earthmagnetism with respect to x axis can be represented by:

θ=tan⁻¹ (V in y direction/V in x direction).

As apparent from the above description, when a direct current is passedin one direction, the intermediate potential output differences at thetime of applications of the biasing magnetic field to the magnetoresistive element pairs 2, 3 and the magneto resistive element pairs 4,5 in x direction and y direction, respectively, can be obtainedsimultaneously, and when a direct current is passed in the oppositedirection, the intermediate potential output differences at the time ofapplications of the biasing magnetic field to the magneto resistiveelement pairs 2, 3 and the magneto resistive element pairs 4, 5 in −xdirection and −y direction, respectively, can be obtainedsimultaneously.

While in the description of EXAMPLE, the angle at which the magnetoresistive thin plates cross the respective sides of the plane coil isassumed to be π/4, that is, 45 degrees, bearings can be measured as faras the angle is more than 0 degree and not more than 60 degrees.However, when the angle is too small, the region in which the resistancevaries according to the magnetic field is reduced in the vicinity of theminimum value in FIG. 4 so that it is difficult to establish anappropriate biasing magnetic field. Therefore, the most manageable angleis 45 degrees.

Furthermore, while in the above EXAMPLE, the cases has been described inwhich the longitudinal directions of the two magneto resistive elementscrossing one side of the plane coil 1 are perpendicular to each other,and the longitudinal directions of the two magneto resistive elements ofone magneto resistive element pair are perpendicular to each other, itis only needed that they are not parallel to each other. However, themost manageable angle thereof is a right angle. It is preferred that theangles at which the magneto resistive elements in one element pair crossthe respective sides is in a mirror image relationship. As therelationship approximates the mirror image relationship, the output isless varied and approximates a sine wave. Therefore, in the magnetoresistive element pair, the difference between the angles at which themagneto resistive elements cross the respective sides falls within therange of +/−5 degrees. More preferably, for all the magneto resistiveelements in the azimuth meter, the variation in the angles at which themagneto resistive elements cross the respective sides falls within therange of +/−5 degrees.

As for the plane coil, when the plane coil having an outer diameter of 2to 3 mm and the number of turns of 50 to 100 turns was manufactured, asufficient output was obtained. The size of the coil is preferably assmall as possible to minimize the power consumption.

The most effective way to generate a required magnetic field by a lowpower supply voltage is to reduce the resistance of the coil. The coilresistance depends on the thickness, width, and length, and the lengthdominantly depends on the size of the coil. While the width andthickness preferably are as large as possible, the thickness is definedby the space between the conductors. Within the restriction of the spacebetween the conductors, the thickness is preferably large. In terms ofmanufacturing, however, it is not preferred that the plating thicknessis too large. Therefore, the adequate thickness is 2 to 5 micrometers.Accordingly, the adequate width is 8 to 20 micrometers.

The distance between the plane coil and the magneto resistive element ispreferably as small as possible because the magnetic field in theextreme vicinity of the plane coil is used in the present invention. Itis suitable that the distance is on the order of 1.5 times the thicknessof the magneto resistive element and wiring film in consideration of theinsulating property of the insulating film disposed therebetween. Theadequate distance is 0.5 to 2 micrometers.

In the EXAMPLES described above, the azimuth meter is of a two-layeredtype in which a magneto resistive element is provided on a substrate anda coil is disposed thereon. The number of the magneto resistive elementsor the coils may be increased. For example, if a three-layered type isprovided in which a magneto resistive element, a coil, and a magnetoresistive element are disposed on a substrate sequentially, the outputcan be doubled. Alternatively, a three-layered type in which a coil, amagneto resistive element, and a coil are disposed on a substratesequentially may be provided. In addition, the magneto resistiveelements may be provided in a plurality of planes parallel to the planein which the plane coil lies.

As a coil configuration, a plane coil of parallelogram, rectangle orcross can be used. Although the present invention has been explained byusing the plane coil of the square shape in the above description, aplane coil of other shape can be used.

In the following, operations of a magneto resistive element and adriving circuit for the magneto resistive element will be described inmore detail.

As described above, FIG. 4 is a graph showing a resistance of a magnetoresistive element and an applied magnetic field at the time when thedirection of a current forms an angle of 45 degrees with the directionof the applied magnetic field. In the present invention, thecharacteristic shown in this drawing is adopted.

FIG. 5 is an explanatory view of operation of a magneto resistiveelement according to the invention. At operating points denoted byreference numerals 61 and 62, the gradients of the curve with respect tothe applied magnetic field, that is, the sensitivities, are larger thanthe gradient of the curve shown in FIG. 20. Therefore, applying themagnetic field at an angle of 45 degrees with respect to thelongitudinal direction of the element (direction of the current in theelement) can provide a high sensitivity. Operations at the pointsdenoted by reference numerals are as follows.

A) Reference numeral 60 denotes a point where a magnetic field equal toor higher than the saturation magnetization Hk is applied in thenegative direction. At this point, the magnetic domain of the magnetoresistive element is aligned in one direction.

B) Reference numeral 61 denotes a point where a bias is applied in thepositive direction. When there is no external magnetic field, theresistance is the value indicated by reference numeral 67, and whenthere is an external magnetic field 65, the resistance is the valueindicated by reference numeral 68.

C) Reference numeral 62 denotes a point where a magnetic field equal toor higher than the saturation magnetization Hk is applied in thepositive direction. The magnetic domain of the magneto resistive elementis aligned in the direction opposite to that for the point 60.

D) Reference numeral 63 denotes a point where a bias is applied in thenegative direction. When there is an external magnetic field 64 (whichis the same as the external magnetic field 65 in direction andmagnitude), the resistance is the value indicated by reference numeral66.

With respect to the resistance value 67 obtained when there is noexternal magnetic field, the resistance changes between the values 66and 68. This difference is denoted by a resistance value 69. Two of fourmagneto resistive elements connected in an MR-bridge configurationoperate in this manner. The remaining two are disposed in the oppositedirection with respect to the external magnetic field and can providesignals having the opposite sign to the resistance value 69 and the samemagnitude as the resistance value 69.

FIG. 6 shows an example of the driving circuit used in the invention.The rhombuses denote MR bridge circuits 201 and 202 constituted by fourmagneto resistive elements. Outputs thereof are amplified by CMOSoperational amplifiers 203 and 204 about by a factor of 100. Given thata current passing through a plane coil 1 in a direction from amultiplexer Y terminal to a multiplexer X terminal is a positivecurrent, FIG. 6 is associated with the timing chart shown in FIG. 7.

A) In FIG. 6, a capacitor 205 in the left frame is charged via aresistance (Isr set) by a sum voltage which is a sum of the absolutevalue of the magnitude of a voltage Vdd (positive) and the absolutevalue of the magnitude of a voltage Vee (negative). Ports A and B arekept at the low level. If a port INH is turned to the low level, themultiplexer X terminal and the multiplexer Y terminal are connected toterminals X0 and Y0, respectively, and a pulse current passes throughthe coil 1 in the direction from the X terminal to the Y terminal.

B) If the port B is turned to the high level, the multiplexer X terminaland the multiplexer Y terminal are connected to terminals X2 and Y2,respectively, and a current passes through the coil 1 in a directionfrom the Y terminal to the X terminal. At this time, the outputs of thetwo MR bridges 201 and 202 are extracted. The port INH is turned to thehigh level to switch off the bias current. During this process, thecapacitor C205 is re-charged.

C) The port A is turned to the high level. Then, if the port INH isturned to the low level, a pulse current passes through the coil 1 in adirection from an X3 terminal to a Y3 terminal.

D) If the port B is turned to the low level, the terminals X1 and Y1 areconnected to each other, and therefore, a current flows in the negativedirection. The outputs of the two MR bridge are extracted again. Then,the port INH is turned to the high level to switch off the coil current.

Subtraction of the values obtained in the two measurements allowsmeasurement of the magnetic field in each direction. FIG. 8 shows avoltage waveform and a coil current waveform for an actual circuit. Thesame operation can be attained if the midway operation is omitted. Atiming chart in such a case is shown in FIG. 9.

Because of the high coil resistance, in the circuit shown in FIG. 6, thepower supply voltage is not enough to provide a sufficient resetcurrent, and thus, another power supply of the opposite polarity isneeded. FIG. 10 shows a circuit improved to address the problem. Whilethe arrangement following the MR bridges shown in FIG. 10 is the same asthat shown in FIG. 6, the method of driving the coil is different fromthat for the circuit shown in FIG. 6. FIG. 11 shows a timing chart forexplaining the operation thereof.

The circuit shown in FIG. 6 requires two power supplies Vdd and Vee. Thecircuit shown in FIG. 10 is improved in this regard. To unite the powersupplies, the circuit is characterized in that a high voltage isobtained with a simple circuit arrangement by superimposing the voltagescharged in the capacitors on the original voltage. The microcomputer isrequired to have two ports (A, B). While the port A is normally kept atthe high level, it is turned to the low level when the sensor is used.The coil 1 is driven via four NOR gates (one IC), and the X terminaloutput serves as the ON/OFF output for power supply for the bridges 201,202 and the operational amplifiers 203, 204. The manners of connectionand operation are the same as in the circuit shown in FIG. 6. The inputof the port B is kept at the low level. Then, the port A is also turnedto the low level, thereby supplying electric power to the amplifiers andthe bridges. The Y terminal side is turned to the low level and the Zterminal side is turned to the high level. From the Z terminal, acurrent flows through a resistor Rs2, the coil 1 and a resistor Rs1 tothe Y terminal. Thus, a voltage is applied to the resistors Rs1, Rs2 andthe coil 1. By this voltage, within several tens microseconds, the plateof a capacitor C1 on the side of the resistor Rs1 is charged negatively,and the plate of a capacitor C2 on the side of the resistor Rs2 ischarged positively. Since the capacitors are provided in a cross(bridge) arrangement between the Y and Z outputs, the sum of theabsolute values of the magnitudes of the voltages of the capacitors isequal to or slightly higher than the power supply voltage.

The input of the port B is turned to the high level. Then, the Yterminal output and the Z terminal output are both inverted. That is,the Y terminal is turned to the high level, and the Z terminal is turnedto the low level. Thus, the plate of the capacitor C2 on the side of theresistor Rs2 connected to the Y terminal is additionally applied withthe previously charged voltage, and thus, the capacitor C2 has a voltageabout 1.5 times higher than the power supply voltage charged therein. Onthe other hand, the capacitor C1 connected to the Z terminal has anegative voltage about 0.5 times higher than the power supply voltageoccurring therein. The coil 1 is applied with the differential voltageof the voltages, that is, a voltage about twice as high as the powersupply voltage. As a result, the coil 1 can produce a sufficientmagnetic field to provide a saturation magnetization of the magnetoresistive elements. This state corresponds to the point 60 in FIG. 5.The capacitors completely discharge in about 2 μs, and then, a currentflows through the coil in a direction from the Y terminal to the Zterminal. The value of the current is associated with the operatingpoint 61 in FIG. 5, and is determined by the resistances of theresistors Rs1, Rs2 and the coil 1 and the power supply voltage. At thistime, the capacitors C1 and C2 are charged in the direction opposite tothat described above. During this process, the output voltages of thetwo MR bridges are amplified and measured. Once the measurement iscompleted, the input of the port B is turned to the low level. Then, theY terminal is turned to the low level, and the Z terminal is turned tothe high level. The charges in the capacitors, having been oppositelycharged, produce a pulse current in the opposite direction. This statecorresponds to the operating point 62 in FIG. 5. Once the discharge iscompleted, a current depending on the resistances of the resistors Rs1,Rs2 and the coil 1 flows through the coil 1 in a direction from the Zterminal to the Y terminal. Then, the operating point 63 is reached. Atthis time, the outputs of the two MR bridges 201, 202 are measuredagain. Once the measurement is completed, the port A is returned to thehigh level. Then, the circuit shown in FIG. 10 stops operating. Afterthat, the differences between the two outputs for the magnetic fields inthe two directions are determined, thereby providing measurements of themagnetic fields. Then, calculation is performed to provide a bearingindication.

FIG. 12 shows measurements of an actual coil current. The significantpositive and negative pulses correspond to the operating points 62 and60 in FIG. 5. The flat positive and negative sections correspond to theoperating points 61, 63. Here, amplified analog voltages are measuredfor each direction four times in total. FIG. 13 is an enlarged graphshowing the initial part of the coil current. The significant negativepulse corresponds to the point 60 in FIG. 5. The coli resistance is 200Ω, the power supply voltage is 3 V, and a current of 23 mA is flowsthrough the coil. The voltage applied to the coil is 4.6 V, which ishigher than the power supply voltage. The pulse width can be changed bythe capacitors C1 and C2. While a larger pulse width is more effective,a larger pulse width requires a longer time to discharge the IC outputand consumes a higher current. Experimentally, an adequate result wasprovided when the capacitors C1 and C2 had a capacitance of 22000 pF andthe capacitors discharging time was about 2 μs.

Further improvement will be described in the following. Sometimes,inversion of the IC output takes a long time. In such a case, thecharges in the capacitors flow through not only the coil, through whichthe current to be passed, but also through the resistors Rs1 and Rs2.This is because the current in the resistor increases in proportion tothe voltage, and thus, the resistors Rs1 and Rs2 may be replaced withbidirectional constant-current elements, which don't increase thecurrent even if the applied voltage increases. Specifically, the circuitis arranged as shown in FIG. 14, and FIG. 15 shows a characteristicthereof. The junction-type field effect transistor used is the 2SK170,and the resistance is 33Ω. There is exhibited a constant currentcharacteristic of about 4 mA. As a result, the peak value of the currentpassing through the IC is 32 mA, while it is about 48 mA when theresistors Rs1 and Rs2 are used.

When the bearing is calculated based on the resulting magnetic fieldstrengths of the earth magnetism in the X direction and the Y direction,an arctangent calculation is required. The principal value of thearctangent is from −90 degrees to +90 degrees in cycles. Therefore, atthe point of switching, the value in the X direction is small, and thus,malfunction tends to occur. Thus, calculation is preferably performed bydividing the function at +/−45 degrees and +/−135 degrees where theX-directional component and the Y-directional component have largeabsolute values.

FIGS. 16 and 17 shows exemplary circuits in the case where an analogoutput is required. These circuits are the same as the circuit describedabove in that the coil 1 is driven by the DFF (D-type flip flop) outputvia a bridge circuit constituted by capacitors C3, C4 and resistors R4,R5, and the operating points 60, 61, 62 and 63 in FIG. 5 are used.However, in this case, after the AD conversion, subtraction cannot beperformed digitally, and thus, subtraction is performed in an analogmanner. That is, the DFF output is connected not only to the coil 1 fordriving thereof but also to the MR bridges 201, 202, and invertedvoltages are applied to the MR bridges 201, 202 so that when the MRbridges 201, 202 provide positive outputs at the operating point 61, anegative output is provided at the operating point 63. In this case, ifthere is no signal, the MR bridges would provide no output. However,normally, the MR bridges produces an offset voltage because of a slightdifference between the resistances thereof. As a result, if the timeratio between the positive and negative voltages applied to the MRbridges is accurately set to 1:1, the reference point of the outputvoltage, which is the average thereof, constitutes the midpoint of thepower supply voltage, and accurate measurement can be attained. In thepresence of an external magnetic field, if a voltage is applied to theMR bridges in the positive direction and the biasing voltage is alsopositive, this state corresponds to the operating point 61, and thus, avoltage associated with the resistance level 68 is produced. If the DFFis inverted, the biasing magnetic field is inverted, resulting in thestate of the operating point 63. However, since the bridge voltage isalso inverted, a voltage associated with the resistance level 66 occursby being inverted. That is, in this drawing, in the presence of apositive external magnetic field, the MR bridge output constantlyprovides a negative voltage independently of the polarity of the voltageapplied to the MR bridge, and the average value (integral value) thereofis also negative. In this way, an analog voltage continuously associatedwith the external magnetic field can be provided.

The azimuth meter according to the invention can provide a mobiledevice, such as a mobile phone or PDA, with a particularly improvednavigation function if it is mounted on the mobile device. In the fieldof mobile phones, application software for displaying a town map, suchas a gourmet guide or hotel guide, has been in practical use. In suchapplication software, in the past, the map has been displayed with theactual bearing and the display screen being fixed. For example, theupper edge of the display screen is associated with the north direction.Thus, if the upper edge of the display screen the mobile phone is notactually directed to the north, the user has to rotate the displayscreen to direct the upper edge thereof to the north, and this is aburdensome procedure for the user. If the azimuth meter according to theinvention is used, there can be provided a mobile device with anavigation function, such as a mobile phone, that can display the map byaligning the bearing of the map displayed on the screen with the actualgeographical without regard to the orientation of the display screen ofthe mobile device. To address an output variation due to an elevationangle, a component for detecting the elevation angle and a correctioncalculation circuit can be additionally provided. Alternatively, theazimuth meter may be a three-dimensional one to address the same. FIG.18 is a perspective view of a mobile phone with a navigation functionincorporating the azimuth meter according to the invention. Referencenumeral 300 denotes a main unit of the mobile phone with a navigationfunction, and reference numeral 310 denotes a liquid crystal display(LCD) unit.

For comparison, a conventional example is shown in FIG. 19. Same partsas in those in FIG. 18 are assigned the same reference numerals. Theposition of the user is indicated by a cross mark 320. The mobile phonewith a navigation function according to the invention shown in FIG. 18can display a map by aligning an actual bearing 400 with a map bearing350. This is achieved by using the bearing information obtained by theincorporated azimuth meter according to the invention. In theconventional example shown in FIG. 19, no small azimuth element can beincorporated in the mobile phone, and thus, the map bearing 350indicating the north is fixed on a display screen 315 and is not alignedwith the actual bearing 400.

INDUSTRIAL APPLICABILITY

As described above in detail, according to the present invention, sincethe plane coil is used to apply the saturation magnetic field andbiasing magnetic field to the magneto resistive elements, the coil canbe made of a thin film, and therefore, the thickness and are of theazimuth meter can be reduced.

With the azimuth meter of the present invention, bearings can bemeasured by applying biasing magnetic fields in x and y directionssimultaneously, so that the number of measurements of bearings can behalved compared to a conventional technique.

In addition, with the process for bearings according to the presentinvention, since a biasing magnetic field of a predetermined strength isapplied in such a manner that the directions of the applied saturationmagnetic field and biasing magnetic field are opposite to each other,the output can be increased.

What is claimed is:
 1. An azimuth meter, comprising: a plane coil woundinto a shape symmetrical with respect to two axes perpendicular to eachother; and at least of groups of thin film magneto resistive elementsdisposed substantially parallel to the plane oft plane coil, in whicheach of said groups of magneto resistive elements constitutes an MRbridge of an even number of magneto resistive elements electricallyconnected to each other and detects and outputs two perpendicularcomponents of the earth magnetism, and bearing information is obtainedbased on the output values, wherein the azimuth meter further comprises:means of passing a current in a predetermined direction through theplane coil to apply thereto a magnetic field that is equal to or higherthan a saturation magnetization of said magneto resistive elements,applying a constant biasing magnetic field in the opposite direction,applying a magnetic field equal to or higher than the saturationmagnetization of the magneto resistive elements in the directionopposite to said predetermined direction, and then applying a biasingmagnetic field in the opposite direction; and means of passing amagnetic field measuring current through the groups of the thin filmmagneto resistive elements concurrently with the applications of saidbiasing magnetic fields.
 2. The azimuth meter according to claim 1,wherein a circuit is arranged so that, when a power supply voltage isused to apply the magnetic field equal to or higher than the saturationmagnetization of the magneto resistive elements, a discharge voltage ofa capacitor having been previously charged by a shunt current from thepower supply is superimposed to the voltage applied to the plane coil.3. The azimuth meter according to claim 2, wherein the variation of anangle at which a longitudinal direction of each magneto resistiveelement intersects the side of a rectangular coil falls within a rangeof ±5 degrees.
 4. The azimuth meter according to claim 1, wherein theplan coil has a rectangular shape, an angle β formed between alongitudinal direction of each magneto resistive element and a side ofthe plane coil intersecting the magneto resistive element satisfies arelation of sin β×cos β≠0, and an applied magnetic field characteristicin the vicinity of a region where the electrical resistance variation ofthe magneto resistive element in response to the applied magnetic fieldis the minimum is used.
 5. The azimuth meter according to claim 4,wherein one of the at least two groups of thin film magneto resistiveelements is constituted by two pairs of magneto resistive elements tothereby constitute said even number of said magneto restrictiveelements, the magneto resistive elements in each pair being disposedintersecting opposite sides of said rectangular coil and beingelectrically connected to each other, the other of the groups of thinfilm magneto resistive elements is constituted by two pairs of magnetoresistive elements, the magneto resistive elements in each pair beingdisposed intersecting opposite sides, different from said sides, of saidrectangular coil and being electrically connected to each other, andwhere the longitudinal directions of the two magneto resistive elementsdisposed on the same side are substantially perpendicular to each other.6. The azimuth meter according to claim 4, wherein the angle β is any ofabout 45 degrees, about 135 degrees, about 225 degrees and about 315degrees.
 7. The azimuth meter according to claim 1, in which the twoperpendicular components of the earth magnetism are detected by each ofthe groups of magneto resistive elements and output therefrom, and thebearing information is obtained based on the output values, wherein acircuit is additionally provided which outputs a difference between anoutput obtained when a bias is applied in a positive direction and anoutput obtained when a bias is applied in a negative direction.
 8. Amethod for obtaining bearing information comprising: providing anazimuth meter which comprises: a plane coil wound into a shapesymmetrical with respect to two axes perpendicular to each other; and atleast two groups of thin film magneto resistive elements disposedsubstantially parallel to the plane of the plane coil, in which each ofsaid groups of magneto resistive elements constitutes an MR bridge of aneven number of magneto resistive elements electrically connected to eachother and detects and outputs two perpendicular components of earthmagnetism, with bearing information being obtained based on outputvalues therefrom; wherein said azimuth meter further comprises: means ofpassing a current in a predetermined direction through said plane coilto apply thereto a magnetic field that is equal to or higher than asaturation magnetization of said magneto resistive elements; applying aconstant biasing magnetic field in the opposite direction; applying amagnetic field equal to or higher than said saturation magnetization ofsaid magneto resistive elements in a direction opposite to saidpredetermined direction, and then applying a biasing magnetic field insaid opposite direction; means for passing a magnetic field measuringcurrent through said groups of the thin film magneto resistive elementsconcurrently with said applications of said biasing magnetic fields,passing said magnetic field measuring current through the plane coil insaid predetermined direction to apply to said groups of thin filmmagneto resistive elements said magnetic field that is equal to orhigher than said saturation magnetization of said magneto resistiveelements; applying to said groups of thin film magnetic resistiveelements said constant biasing magnetic field in said oppositedirection, while passing said magnetic field measuring current throughsaid groups of thin film magneto resistive elements to measure aresulting magnetic field and to obtain an output value from said groupsof thin film magneto resistive elements; applying to said groups of thinfilm magneto resistive elements said magnetic field equal to or higherthan said saturation magnetization of said magneto resistive elements insaid direction opposite to said predetermined direction, applying tosaid groups of thin film magneto resistive elements said biasingmagnetic field in said direction opposite to said direction of saidmagnetic field applied immediately before, which magnetic field appliedimmediately before is said magnetic field which is equal to or higherthan said saturation magnetization of said magneto restrictive elementswhile passing said magnetic field measuring current through said groupsof thin film magneto resistive elements to measure said resultingmagnetic field and to obtain an output value from said groups of thinfilm magneto resistive elements, and obtaining bearing information onsaid output values.